Binder composition for magnetic recording medium and magnetic recording medium

ABSTRACT

A binder composition for a magnetic recording medium contains a vinyl copolymer having structural units denoted by each of formulas [1], [2] and [3]. In formula [1], R 1  is hydrogen, halogen, or methyl, L 1  is a single bond or a divalent linking group, and Y is an alicyclic group. In formula [2], R 2  is hydrogen, halogen, or methyl, L 2  is a single bond or a divalent linking group, and Z is a hydrocarbon group with a carbon number of from 8 to 50. In formula [3], R 3  is hydrogen, halogen, or methyl, and L 3  is a single bond or a divalent linking group:

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 USC 119 toJapanese Patent Application No. 2010-083628, filed on Mar. 31, 2010,which is expressly incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a binder composition for a magneticrecording medium. More particularly, the present invention relates to abinder composition for a magnetic recording medium containing a binderresin that is suited to the manufacturing of magnetic recording mediahaving good electromagnetic characteristics.

The present invention further relates to a magnetic recording mediumcontaining the above binder resin.

2. Discussion of the Background

In particulate magnetic recording media, the binder plays importantroles with respect to electromagnetic characteristics.

Vinyl chloride resin, polyurethane resin, polyester resin, acrylicresin, and various other resins are employed as binders in magneticrecording media. Of these, vinyl polymers such as vinyl chloride resinand acrylic resin are widely employed due to the high degree of unitfreedom, ease of conducting the synthesis reaction, and the like theyafford (see Document 1: Japanese Unexamined Patent Publication (KOKAI)Heisei No. 8-67855; Document 2: Japanese Unexamined Patent Publication(KOKAI) No. 2004-295926; and Document 3: Japanese Unexamined PatentPublication (KOKAI) Heisei No. 6-111277, which are expresslyincorporated herein by reference in their entirety).

In addition to using microparticulate magnetic material, increasing thesmoothness of the magnetic layer surface by dispersing microparticulatemagnetic material to a high degree is an effective way of achievinghigh-density recording in the field of magnetic recording. Further,increasing the dispersion of the nonmagnetic powder contained in thenonmagnetic layer positioned beneath the magnetic layer is an effectiveway to increase the smoothness of the magnetic layer surface.Accordingly, the introduction of adsorptive functional groups (polargroups) such as SO₃Na groups into the binders of magnetic recordingmedia is widely practiced. However, when the quantity of polar groupsintroduced into the binder is increased to increase dispersion of themicroparticulate powder, there is a risk of association between polargroups conversely causing a decrease in dispersion. Accordingly, itbecomes difficult to ensure adequate dispersion by simply introducingpolar groups. That is, it is difficult to achieve higher densityrecording by simply introducing polar groups into conventional vinylpolymers, including the polymers described in Documents 1 to 3.

SUMMARY OF THE INVENTION

An aspect of the present invention provides for a binder for use inmagnetic recording media that is comprised of a vinyl polymer andpermits the manufacturing of magnetic recording media for use inhigh-density recording that have good electromagnetic characteristics.

The present inventors conducted extensive research into achieving theabove binder, resulting in the discovery that a vinyl polymer having thestructural unit denoted by general formula [G] below and the structuralunit denoted by general formula [P] below permitted a high degree ofdispersion of ultrafine particulate powder. This was attributed to therepeating unit (the repeating unit denoted by general formula [a])having a graft chain structure contained in general formula [G] and thepolar group contained in general formula [P] contributing to dispersion.This point will be described in greater detail. The present inventorssurmised that effective spreading of the graft chain structure containedin general formula [a] within the dispersion medium inhibited theaggregation of particles, and that the polar groups contained in generalformula [P] adsorbed to the surface of the particles, increasingdispersion of the microparticulate powder within the coating liquid.

The present invention was devised based on these discoveries.

An aspect of the present invention relates to a binder composition for amagnetic recording medium, which comprises a vinyl copolymer comprisinga structural unit denoted by general formula [G] and a structural unitdenoted by general formula [P]:

wherein, in general formula [G], R^(a1) denotes a hydrogen atom, ahalogen atom, or a methyl group, L^(a) denotes a single bond or adivalent linking group, and A denotes a repeating unit denoted bygeneral formula [a]:

wherein, in general formula [a], each of R^(a11), R^(a12), and R^(a13)independently denotes a hydrogen atom, a halogen atom, or a methylgroup, each of A¹ and A² independently denotes a vinyl monomer residue,n1 denotes an integer of equal to or greater than 1, and * denotes aposition of a bond with the group denoted by L^(a);

wherein, in general formula [P], each of R^(b1) and R^(b2) independentlydenotes a hydrogen atom, a halogen atom, or a methyl group, each ofL^(b1) and L^(b2) independently denotes a single bond or a divalentlinear linking group, each of X^(b1) and X^(b2) denotes a hydrogen atom,a halogen atom, a methyl group, or a polar group selected from the groupconsisting of a sulfonic acid group, a sulfonate group, a carboxylicacid group, a carboxylate group, a phosphoric acid group, and aphosphate group, wherein at least either X^(b1) or X^(b2) is the polargroup.

The repeating unit denoted by general formula [a] may be a repeatingunit denoted by general formula [a1]:

wherein, in general formula [a1], each of R^(a11), R^(a12), R^(a13),and * is defined as in general formula [a], each of A¹¹ and A¹²independently denotes a (meth)acrylate monomer residue, and n2 denotesan integer of equal to or greater than 10 but equal to or less than 500.

The vinyl copolymer may further comprise a structural unit denoted bygeneral formula [1], a structural unit denoted by general formula [2],and a structural unit denoted by general formula [3]:

wherein, in general formula [1], R¹ denotes a hydrogen atom, a halogenatom, or a methyl group, L¹ denotes a single bond or a divalent linkinggroup, and Y an alicyclic group;

wherein, in general formula [2], R² denotes a hydrogen atom, a halogenatom, or a methyl group, L² denotes a single bond or a divalent linkinggroup, and Z denotes a hydrocarbon group with a carbon number rangingfrom 8 to 50;

wherein, in general formula [3], R³ denotes a hydrogen atom, a halogenatom, or a methyl group, and L³ denotes a single bond or a divalentlinking group.

The structural unit denoted by general formula [3] may be a structuralunit denoted by general formula [6]:

wherein, in general formula [6], R³¹ denotes a hydrogen atom or a methylgroup, X³ denotes —O—, —S—, or a divalent linking group denoted by—N(R³³)—, R³³ denotes a hydrogen atom or an optionally substituted alkylgroup with a carbon number ranging from 1 to 8, and R³² denotes anoptionally substituted alkylene group with a carbon number ranging from2 to 8 or a divalent group in which multiple such alkylene groups arelinked through a linking group.

The structural unit denoted by general formula [1] may be a structuralunit denoted by general formula [4]:

wherein, in general formula [4], R¹¹ denotes a hydrogen atom or a methylgroup, X¹ denotes —O—, —S—, or a divalent linking group denoted by—N(R¹²)—, R¹² denotes a hydrogen atom or an optionally substituted alkylgroup with a carbon number ranging from 1 to 8, and Y¹ denotes analicyclic condensed cyclic group.

The structural unit denoted by general formula [2] may be a structuralunit denoted by general formula [5]:

wherein, in general formula [5], R²¹ denotes a hydrogen atom or a methylgroup, X² denotes a divalent linking group denoted by —(O)m¹-, —(S)m²-,or —{N(R²²)}m³-, each of m¹, m², and m³ independently denotes an integerof equal to or greater than 1, R²² denotes an optionally substitutedalkyl group with a carbon number ranging from 1 to 8, and n denotes aninteger ranging from 12 to 30.

The binder composition may further comprise a polyisocyanate.

A further aspect of the present invention relates to a magneticrecording medium comprising a magnetic layer comprising a ferromagneticpowder and a binder on a nonmagnetic support, which comprises at leastone layer comprising a binder of which constituent component is theabove vinyl copolymer.

The above layer may comprise a reaction product of the vinyl copolymerand a polyisocyanate.

The above layer may be the magnetic layer.

The above layer may be a nonmagnetic layer comprising a nonmagneticpowder and a binder and being positioned between the magnetic layer andthe nonmagnetic support.

The ferromagnetic powder may be a hexagonal ferrite powder having anaverage plate diameter ranging from 10 nm to 50 nm.

The ferromagnetic powder may be a ferromagnetic metal powder having anaverage major axis length ranging from 20 nm to 50 nm.

The present invention can provide a magnetic recording medium thataffords good electromagnetic characteristics in high-density recording.

Other exemplary embodiments and advantages of the present invention maybe ascertained by reviewing the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Unless otherwise stated, a reference to a compound or component includesthe compound or component by itself, as well as in combination withother compounds or components, such as mixtures of compounds.

As used herein, the singular forms “a,” “an,” and “the” include theplural reference unless the context clearly dictates otherwise.

Except where otherwise indicated, all numbers expressing quantities ofingredients, reaction conditions, and so forth used in the specificationand claims are to be understood as being modified in all instances bythe term “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that may vary depending upon thedesired properties sought to be obtained by the present invention. Atthe very least, and not to be considered as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should be construed in light of the number ofsignificant digits and ordinary rounding conventions.

Additionally, the recitation of numerical ranges within thisspecification is considered to be a disclosure of all numerical valuesand ranges within that range. For example, if a range is from about 1 toabout 50, it is deemed to include, for example, 1, 7, 34, 46.1, 23.7, orany other value or range within the range.

The following preferred specific embodiments are, therefore, to beconstrued as merely illustrative, and non-limiting to the remainder ofthe disclosure in any way whatsoever. In this regard, no attempt is madeto show structural details of the present invention in more detail thanis necessary for fundamental understanding of the present invention; thedescription making apparent to those skilled in the art how severalforms of the present invention may be embodied in practice.

Binder Composition

The binder composition for a magnetic recording medium of the presentinvention comprises a vinyl copolymer comprising a structural unitdenoted by general formula [G] and a structural unit denoted by generalformula [P]. The above vinyl copolymer is also referred to as “thebinder of the present invention”, hereinafter. As stated above, thebinder of the present invention, in the form of a vinyl copolymercontaining the above two structural units, can disperse microparticulatepowder to a high degree and thus enhance the surface smoothness of themagnetic layer.

The binder of the present invention will be described in greater detailbelow. The structural unit denoted by general formula [G] will also bereferred to as structural unit [G] below. The same applies to thestructural units denoted by the other general formulas. The structuralunit [G] contained in the binder of the present invention may be of asingle type, or may comprised of two or more different types. The sameapplies to the other structural units.

Structural Unit [G]

As set forth above, the binder of the present invention has a graftchain structure in general formula [G]. In this regard, as relates tograft copolymers suited to magnetic recording media, Japanese UnexaminedPatent Publication (KOKAI) Heisei No. 7-9696, which is expresslyincorporated herein by reference in its entirety, discloses a graftcopolymer having a perfluoroalkyl group. However, the graft copolymerdescribed in this publication is employed as a lubricant; which doesn'thave properties required for a binder.

General formula [G] will be described in detail below.

In general formula [G], R^(a1) denotes a hydrogen atom, a halogen atom,or a methyl group, desirably denoting a hydrogen atom or a methyl group.Examples of halogen atoms are chlorine, bromine, and iodine atoms.

L^(a) denotes a single bond or a divalent linking group, with a linkinggroup comprising any from among oxygen atoms, nitrogen atoms, and sulfuratoms being desirable. In terms of synthesis, a divalent linking groupthat is bonded through —C(O)— to a carbon atom on the main chain, or adivalent linking group that is bonded to a group denoted by A belowthrough a sulfur atom, is desirable. A linking group with three to eightconnected divalent groups selected from the group consisting of —O—,—S—, —N—, —C(═O)—, —NHC(═O)— alkylene groups with 2 to 4 carbon atoms,and arylene groups with 6 to 10 carbon atoms is preferred from theperspective of enhancing dispersion.

In general formula [G], A denotes a repeating unit denoted by generalformula [a]. As stated above, the presence of the graft chain structuredenoted by general formula [a] in the binder of the present invention isthought to play a role in enhancing dispersion.

In general formula [a], each of R^(a11), R^(a12), and R^(a13)independently denotes a hydrogen atom, a halogen atom, or a methylgroup. The details are identical to those set forth for R^(a1). Ingeneral formula [a], the “*” denotes the position of the bond with thegroup denoted by L^(a).

In general formula [a], each of A¹ and A² independently denotes a vinylmonomer residue. Examples of vinyl monomer residues are the varioussubstituents contained in vinyl monomers given by way of example forcopolymerizable monomers. Of these, a —OCOR group, —C(═O)OR group (Rbeing an alkyl group), —C(═O)OR′OH (R′ being an alkylene group), arylgroup (desirably an aryl group with 6 to 10 carbon atoms), and cyanogroup are desirable. An alkyl group with 1 to 8 carbon atoms isdesirable and a methyl group is preferable as the alkyl groups denotedby R and R′.

In general formula [a], n1 denotes an integer of equal to or greaterthan 1. From the perspective of enhancing dispersion, an integer ofequal to or greater than 10 is desirable, and from the perspective ofsolubility, an integer of equal to or less than 500 is desirable.

Since n1 denotes an integer of equal to or greater than 1 in generalformula [a], two or more structural units with branched structures arepresent in the binder of the present invention. However, thesestructural units do not have to be identical; different structures maybe incorporated.

From the perspective of enhancing dispersion, the repeating unit denotedby general formula [a] is desirably denoted by general formula [a1]below.

In general formula [a1], each of R^(a11), R^(a12), R^(a13), and * isdefined as in general formula [G] and the details are as set forthabove.

In general formula [a1], each of A¹¹ and A¹² independently denotes a(meth)acrylate monomer residue. Examples of (meth)acrylate monomerresidues are the various substituents contained in the (meth)acrylatemonomers given by way of example for copolymerizable monomers. Alkylgroups are desirable, alkyl groups having 1 to 8 carbon atoms arepreferred, and methyl groups are of greater preference as thesesubstituents.

In general formula [a1], n2 denotes an integer of equal to or greaterthan 10 but equal to or less than 500. From the perspective of achievingboth dispersion and solubility, an integer of equal to or greater than20 but equal to or less than 100 is desirable.

Structural unit [G] set forth above can be derived from the vinylmonomer denoted by general formula [G′] below.

(In general formula [G′], each of R^(a1), L^(a), and A is defined as ingeneral formula [G].])

Specific examples of the vinyl monomer (graft chain monomer) denoted bygeneral formula [G′] will be given below. However, the present inventionis not limited to the specific examples given below.

Structural Unit [P]

In the binder of the present invention, a functional group (polar group)that is capable of adsorbing to the surface of the magnetic powder andnonmagnetic powder in a particulate magnetic recording medium is presentin structural unit [P]. The presence of both structural unit [P] andstructural unit [G] can increase the dispersion of the powder in thecoating liquid.

In general formula [P], each of R^(b1) and R^(b2) independently denotesa hydrogen atom, a halogen atom, or a methyl group. The details are asdescribed above for R^(a1) and the like in general formula [G].

Each of L^(b1) and L^(b2) independently denotes a single bond or adivalent linear linking group. That is, the linking group does notcontain a graft chain structure such as that contained in structuralunit [G]. This is because a structure containing a graft chain in thelinking group causes adsorption between particles of magnetic powder andnonmagnetic powder, compromising dispersion. From the perspective ofenhancing dispersion, L^(b1) and L^(b2) are desirably single bonds ordivalent linear linking groups bonded through —C(O)— groups to carbonatoms on the main chain.

Each of X^(b1) and X^(b2) denotes a hydrogen atom, a halogen atom(chlorine atom, bromine atom, iodine atom, or the like), a methyl group,or a polar group selected from the group consisting of a sulfonic acid(salt) group, a carboxylic acid (salt) group, and a phosphoric acid(salt) group. In the present invention, the term “sulfonic acid (salt)group” includes the sulfonic acid group (—SO₃H), sulfonate groups suchas the SO₃Na group, SO₃K group, and SO₃Li group, and salts thereof. Thesame applies to carboxylic acid (salt) groups and phosphoric acid (salt)groups. In structural unit [P], at least either X^(b1) or X^(b2)contains the polar group. This group adsorbs to the powder, enhancingdispersion of the powder. From the perspective of inhibiting associationof polar groups, either X^(b1) or X^(b2) desirably contains the polargroup.

To further enhance dispersion, the quantity of the polar group that isincorporated into the binder of the present invention is desirably 10 to1,000 μeq/g.

Structural unit [P] as set forth above can be derived from the vinylmonomer denoted by general formula [P′] below.

(In general formula [P′], each of R^(b1), R^(b2), L^(b1), L^(b2),X^(b1), and X^(b2) is defined as in general formula [P].])

Specific examples of monomers denoted by general formula [P′] containingsulfonic acid (salt) groups are given below. However, the presentinvention is not limited to the following specific examples.

(In the above, M denotes a hydrogen atom, an alkali metal atom, or anammonium salt, and R denotes an alkyl group.)

Examples of monomers denoted by general formula [P′] containingcarboxylic acid (salt) groups are unsaturated carboxylic acids such as(meth)acrylic acid, maleic acid, and itaconic acid; unsaturatedcarboxylic anhydrides such as maleic anhydride and itaconic anhydride;and half-esters thereof.

Examples of monomers denoted by general formula [P′] containingphosphoric acid (salt) groups are: monomers comprising monophosphoricacid (salt) groups, such as mono[(meth)acryloyloxyethyl] acid phosphate,mono[(meth)acryloyloxypropyl] acid phosphate,mono[(meth)acryloyloxybutyl] acid phosphate,mono[(meth)acryloyloxyethoxynoethyl] acid phosphate,mono[(meth)acryloyloxypolyoxyethyleneglycol] acid phosphate,(meth)acryloyloxyethylmethyl acid phosphate, (meth)acryloyloxyethylbutylacid phosphate, (meth)acryloyloxypropylethyl acid phosphate,(meth)acryloyloxyethylmethyl acid phosphate,(meth)acryloyloxypolyoxyethyleneglycol butyl acid phosphate, vinyl acidphosphate, and alkali metal salts thereof. The above (meth)acrylic acidincludes methacrylic acid and acrylic acid. The term “(meth)acryloyl”refers to methacryloyl and acryloyl. The “(meth)acrylate” in the presentinvention referred to below includes methacrylate and acrylate.

From the perspective of achieving good dispersion, the binder of thepresent invention in the form of a vinyl copolymer desirably containsequal to or greater than 0.1 molar percent but equal to or less than 75molar percent, preferably contains equal to or greater than 1 molarpercent but equal to or less than 50 molar percent, and more preferably,contains equal to or greater than 1 molar percent but equal to or lessthan 30 molar percent of structural unit [G] based on all polymerizationunits constituting the copolymer.

Similarly, from the perspective of achieving good dispersion, the binderof the present invention desirably contains equal to or greater than0.01 molar percent but equal to or less than 25 molar percent,preferably contains equal to or greater than 0.1 molar percent but equalto or less than 10 molar percent, and more preferably, contains equal toor greater than 0.1 molar percent but equal to or less than 5 molarpercent of structural unit [P]. Accordingly, the blending ratio of thevarious monomers during the polymerization reaction is desirablyestablished to obtain a copolymer having the above desirablecomposition. The quantity of the above-described polar groupincorporated into the binder of the present invention is desirably 10 to1,000 μeq/g.

Structural Units that can Also be Present

The binder of the present invention can be comprised of just structuralunits [G] and [P]. However, other structural units can be incorporatedas needed to achieve a magnetic recording medium binder with moredesirable physical properties. Examples of such structural units thatcan also be present are the structural unit denoted by general formula[1] below, the structural unit denoted by general formula [2] below, andthe structural unit denoted by general formula [3] below. The jointpresence of these structural units can further enhance dispersion andimprove running durability by further increasing the coating strength.This is primarily attributed to the alicyclic group contained in generalformula [1], the long chain or polycyclic hydrocarbon group contained ingeneral formula [2], and the hydroxyl group contained in general formula[3] contributing to dispersion, and the hydroxyl group contained ingeneral formula [3] crosslinking with polyisocyanate to enhance coatingstrength.

These structural units that can also be present will be described ingreater detail below.

Structural Unit [1]

In general formula [1], R¹ denotes a hydrogen atom, a halogen atom, or amethyl group. Examples of halogen atoms are chlorine atoms, bromineatoms, and iodine atoms.

R¹ desirably denotes a hydrogen atom or a methyl group, and preferablydenotes a methyl group.

In general formula [1], L¹ denotes a single bond or a divalent linkinggroup. The divalent linking group denoted by L¹ desirably contains ahetero atom, it being desirable for the hetero atom to bond with thealicyclic group denoted by Y. Examples of the hetero group are an oxygenatom, nitrogen atom, and sulfur atom. L¹ desirably denotes a single bondor a divalent linking group that bonds through a —C(O)— group to acarbon atom of the main chain, and preferably denotes a divalent linkinggroup denoted by —C(O)X¹— in general formula [4] described furtherbelow.

In general formula [1], Y denotes an alicyclic group. The alicyclicgroup may be saturated or unsaturated, and may be monocyclic,polycyclic, or a condensed ring. It may also comprise a substituent.

In the present invention, when a given group contains a substituent,examples of the substituent are an alkyl group (such as an alkyl grouphaving 1 to 6 carbon atoms), a hydroxyl group, an alkoxyl group (such asan alkoxyl group having 1 to 6 carbon atoms), a halogen atom (such as afluorine atom, chlorine atom, or bromine atom), cyano group, aminogroup, nitro group, acyl group, and carboxyl group. For the groupcomprising a substituent, the “carbon number” or the “number of carbonatoms” means the number of carbon atoms of the moiety without thesubstituent. In the present invention, the word “to” between numbersindicates a range that includes the preceding and succeeding numbers asthe minimum value and maximum value thereof, respectively.

The monocyclic alicyclic group denoted by Y is desirably five orsix-membered. Specific examples are a cyclohexyl group and a cyclopentylgroup. In the case of a polycyclic alicyclic group, it is desirablyseven to ten-membered. Specific examples are a bicycloalkyl group,adamantyl group, norbornyl group, and isobornyl group. From theperspective of enhancing dispersibility, Y desirably denotes analicyclic condensed cyclic group. The alicyclic condensed ring isdesirably the alicyclic condensed ring denoted by Y¹ in general formula[4], described further below.

From the perspective of enhancing dispersibility, structural unit [1]desirably denotes the structural unit (structural unit [4]) denoted bygeneral formula [4] below.

In general formula [4], R¹¹ denotes a hydrogen atom or a methyl group,preferably a methyl group.

X¹ denotes —O—, —S—, or a divalent linking group denoted by —N(R¹²)—,with —O— being preferred.

R¹² denotes a hydrogen atom or an optionally substituted alkyl groupwith a carbon number ranging from 1 to 8. The carbon number of the alkylgroup denoted by R¹² desirably ranges from 1 to 4. Keeping the number ofcarbon atoms of the alkyl group denoted by R¹² within this range canmake it possible to further enhance dispersibility while maintainingsolubility.

Y¹ denotes an alicyclic condensed cyclic group. The alicyclic condensedcyclic group denoted by Y¹ is desirably seven to ten-membered. Specificexamples of desirable condensed cyclic groups are adamantyl groups,norbornyl groups, and dicyclopentanyl groups.

Above-described structural unit [1] can be derived from the vinylmonomer denoted by general formula [1′] below, and structural unit [4]can be derived from the acrylic monomer denoted by general formula [4′]below.

(In general formula [1′], each of R¹, L¹, and Y is defined as in generalformula [1].)

(In general formula [4′], each of R¹¹, X¹, and Y¹ is defined as ingeneral formula [4].)

Specific examples of the monomers denoted by general formula [1′] andgeneral formula [4′] are given below. However, the present invention isnot limited thereto.

Structural Unit [2]

In general formula [2], R² denotes a hydrogen atom, a halogen atom, or amethyl group. The details are as set forth for R¹ in general formula [1]above.

In general formula [2], L² denotes a single bond or a divalent linkinggroup, with a linking group comprising an oxygen atom, nitrogen atom, orsulfur atom being desirable. A single bond or a divalent linking groupbonded through a —C(O)— group to a carbon atom on the main chain isdesirable as L², and the divalent linking group denoted by —C(O)X²— ingeneral formula [5] described further below is preferred.

In general formula [2], Z denotes a hydrocarbon group with a carbonnumber ranging from 8 to 50. The hydrocarbon group is a saturated orunsaturated linear, branched, or cyclic saturated or unsaturatedhydrocarbon group. A linear or branched hydrocarbon group is preferred.Having a carbon number of equal to or more than 8 can make it possibleto contribute to dispersibility, and having a carbon number of equal toor less than 50 can ensure solubility. From the perspectives ofdispersibility and solubility, the carbon number of the hydrocarbongroup desirably ranges from 12 to 30. The hydrocarbon group denoted by Zis preferably an alkyl group with a carbon number ranging from 12 to 30,and is preferably an alkyl group with a carbon number ranging from 12 to18.

From the perspective of enhancing dispersibility, structural unit [2] isdesirably the structural unit denoted by general formula [5] below(structural unit [5]).

In general formula [5], R²¹ denotes a hydrogen atom or a methyl group,and desirably denotes a methyl group.

n denotes an integer ranging from 12 to 30, preferably an integerranging from 12 to 18.

X² denotes a divalent linking group represented by —(O)m¹-, —(S)m²-, or—{N(R²²)}m³-, and desirably denotes the divalent linking grouprepresented by —(O)m¹-. Each of m¹, m², and m³ independently denotes aninteger of equal to or greater than 1.

R²² denotes an optionally substituted alkyl group with a carbon numberranging from 1 to 8. The carbon number of the alkyl group denoted by R²²desirably ranges from 1 to 4. Keeping the number of carbon atoms of thealkyl group denoted by R²² within this range can further enhancedispersibility while maintaining solubility.

From the perspective of maintaining solubility, each of m¹, m², and m³desirably denotes an integer of equal to or lower than 5.

Above-described structural unit [2] can be derived from the vinylmonomer denoted by general formula [2′] below, and structural unit [5]can be derived from the acrylic monomer denoted by general formula [5′]below.

(In general formula [2′], each of R², L², and Z is defined as in generalformula [2].)

(In general formula [5′], each of R²¹, X², and n is defined as ingeneral formula [5].)

Specific examples of the monomers denoted by general formula [2′] andgeneral formula [5′] are given below. However, the present invention isnot limited thereto.

Structural Unit [3]

In general formula [3], R³ denotes a hydrogen atom, a halogen atom, or amethyl group. The details are as set forth for R¹ in general formula [1]above.

In general formula [3], L³ denotes a single bond or a divalent linkinggroup, with a linking group comprising an oxygen atom, nitrogen atom, orsulfur atom being preferred. Since structural unit [1], structural unit[2], and structural unit [3] are mutually different structures, the L³in general formula [3] does not contain groups corresponding to Y and Zdescribed above. The divalent linking group denoted by L³ is desirablyone that is bonded through a —C(O)— group to a carbon atom on the mainchain, and is preferably the divalent linking group denoted by—C(O)X³R³²— in general formula [6] described further below.

From the perspective of enhancing dispersibility, structural unit [3] isdesirably the structural unit denoted by general formula [6] below(structural unit [6]).

In general formula [6], R³¹ denotes a hydrogen atom or a methyl group,desirably a methyl group.

X³ denotes —O—, —S—, or the divalent linking group denoted by —N(R³³)—,desirably —O—.

R³³ denotes a hydrogen atom or an optionally substituted alkyl groupwith a carbon number ranging from 1 to 8. The carbon number of the alkylgroup denoted by R³³ desirably ranges from 1 to 4. Keeping the number ofcarbon atoms of the alkyl group denoted by R³³ to within this range canfurther enhance dispersibility while maintaining solubility.

R³² denotes an optionally substituted alkylene group with a carbonnumber ranging from 2 to 8 or a divalent group in which multiple suchalkylene groups are linked through linking groups. Keeping the number ofcarbon atoms of the alkylene group contained in the group denoted by R³²to within this range can further enhance dispersibility whilemaintaining solubility. The linking group linking the alkylene group isdesirably an ester bond from the perspective of solubility. The numberof alkylene groups with a carbon number ranging from 2 to 8 that arecontained in the group denoted by R³² is desirably equal to or more than1 but equal to or less than 3.

Above-described structural group [3] can be derived from the vinylmonomer denoted by general formula [3′] below, and structural unit [6]can be derived from the acrylic monomer denoted by general formula [6′]below.

(In general formula [3′], each of R³ and L³ is defined as in generalformula [3].)

(In general formula [6′], each of R³¹, R³², and X³ is defined as ingeneral formula [3].)

The following monomers are specific examples of monomers denoted bygeneral formula [3′] or [6′] (however, the present invention is notlimited to these specific examples): hydroxyalkyl(meth)acrylates such ashydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate, polyethyleneglycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate,polyethylene glycol polypropylene glycol mono(meth)acrylate, glycerolmono(meth)acrylate, and 3-chloro-2-hydroxypropyl(meth)acrylate; vinylethers such as hydroxyethyl vinyl ether, hydroxypropyl vinyl ether, andhydroxybutyl vinyl ether; (meth)allyl ethers such as hydroxyethylmono(meth)allyl ether, hydroxypropyl mono(meth)allyl ether, hydroxybutylmono(meth)allyl ether, diethylene glycol mono(meth)allyl ether,dipropylene glycol mono(meth)allyl ether, glycerin mono(meth)allylether, and 3-chloro-2-hydroxypropyl(meth)allyl ether; and (meth)allylalcohol.

In the binder of the present invention, the hydroxyl group contained instructural unit [3] is capable of forming a crosslinked structure with acrosslinking agent such as polyisocyanate employed as a curing agent inmagnetic recording media. Thus, a magnetic recording medium of highcoating strength can be obtained. The details of the polyisocyanatecuring agent employed are given further below.

From the perspective of achieving both good dispersion and high coatingstrength, the binder of the present invention in the form of a vinylcopolymer desirably comprises equal to or greater than 5 molar percentbut equal to or less than 75 molar percent, preferably comprises equalto or greater than 15 molar percent but equal to or less than 60 molarpercent, and more preferably, comprises equal to or greater than 30molar percent but equal to or less than 50 molar percent of structuralunit [1] (desirably structural unit [4]) based on all polymerizationunits constituting the copolymer.

Similarly, from the perspective of achieving both good dispersion andhigh coating strength, the binder of the present invention desirablycomprises equal to or greater than 5 molar percent but equal to or lessthan 75 molar percent, preferably equal to or greater than 5 molarpercent but equal to or less than 50 molar percent, and more preferably,equal to or greater than 10 molar percent but equal to or less than 30molar percent of structural unit [2] (desirably structural unit [5]),and desirably comprises equal to or greater than 5 molar percent butequal to or less than 80 molar percent, preferably equal to or greaterthan 15 molar percent but equal to or less than 70 molar percent, andmore preferably, equal to or greater than 30 molar percent but equal toor less than 60 molar percent of structural unit (3) (desirablystructural unit [6]) based on all polymerization units. Accordingly, theblending ratio of each monomer during the polymerization reaction isdesirably established to obtain a copolymer having the above desirablecomposition.

Other than the above monomers for introducing the structural units thatcan also be present, examples of monomers that are copolymerizable withthe monomers to obtain the binder of the present invention are ethylenicunsaturated carboxylic ester monomers, aromatic vinyl monomers,ethylenic unsaturated nitrile monomers, ethylenic unsaturated acidmonomers, alkyl vinyl ether monomers, vinyl ester monomers, andethylenic unsaturated polyvalent carboxylic anhydrides.

Examples of ethylenic unsaturated carboxylic ester monomers arealkyl(meth)acrylate monomers and alkoxyalkyl(meth)acrylate monomers.

Examples of alkyl(meth)acrylate monomers are: methyl(meth)acrylate,ethyl(meth)acrylate, propyl(meth)acrylate, butyl(meth)acrylate,isobutyl(meth)acrylate, pentyl(meth)acrylate, hexyl(meth)acrylate,octyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, lauryl(meth)acrylate,stearyl(meth)acrylate, and cyclohexyl(meth)acrylate. Of these,methyl(meth)acrylate is desirable.

The alkyl group of the alkyl(meth)acrylate monomer may comprise asubstituent, and is desirably an aralkyl group. Specific examples ofsuch compounds are: benzyl(meth)acrylate, phenoxyethyl(meth)acrylate,phenoxypolyethylene glycol(meth)acrylate, and nonylphenolethylene oxideadduct (meth)acrylate. Of these, benzyl(meth)acrylate andphenoxyethyl(meth)acrylate are desirable.

Examples of alkoxyalkyl(meth)acrylate monomers aremethoxyethyl(meth)acrylate and butoxyethyl(meth)acrylate. An example ofan ethylenic unsaturated carboxylic ester monomer isglycidyl(meth)acrylate.

Examples of aromatic vinyl monomers are styrene, α-methyl styrene, vinyltoluene, monochlorostyrene, p-methyl styrene, and hydroxymethyl styrene.

Examples of ethylenic unsaturated nitrile monomers are acrylonitrile,methacrylonitrile, 2-ethylpropenenitrile, 2-propylpropenenitrile,2-chloropropenenitrile, and 2-butenenitrile.

Examples of alkyl vinyl ether monomers are allyl glycidyl ether, methylvinyl ether, ethyl vinyl ether, isobutyl vinyl ether, n-butyl vinylether, 2-ethylhexyl vinyl ether, n-octyl vinyl ether, lauryl vinylether, cetyl vinyl ether, and stearyl vinyl ether.

Examples of vinyl ester monomers are vinyl formate, vinyl acetate, vinylpropionate, isopropenyl acetate, vinyl valerate, vinyl caprate, vinyllaurate, vinyl stearate, vinyl benzoate, vinyl versatate, and vinylpivalate.

Examples of ethylenic unsaturated polyvalent carboxylic anhydrides aremaleic anhydride and itaconic anhydride.

The above copolymerizable monomers can be employed singly or incombinations of two or more.

Of these monomers, the ethylenic unsaturated carboxylic acid monomersare desirable and the alkyl(meth)acrylate monomers are preferred.

The various monomers used to obtain the binder of the present inventioncan all be synthesized by known methods or are available as commercialproducts. For example, the vinyl monomer denoted by general formula [G′]for introducing structural unit [G] can be manufactured readily and athigh yield by known methods such as the method described on page 57 ofMicromonomer Chemistry and Industry (IPC Publishing Department), whichis expressly incorporated herein by reference in its entirety.

To obtain the binder of the present invention, it is desirable to employa known polymerization method such as solution polymerization topolymerize the polymerization reaction system optionally including thecopolymerizable compounds as set forth above.

From the perspective of reactivity, it is desirable to employ awater-miscible polar solvent in solution polymerization. In the presentinvention, the term “water-miscible polar solvent” refers to a solventthat dissolves equal to or more than 5 weight percent of water at 20° C.Specific examples of such solvents are dimethylformamide (DMF),dimethylacetamide (DMAC), and N-methylpyrrolidone (NMP). Thepolymerization reaction can be conducted in the presence of a knownpolymerization initiator, chain transfer agent, or the like. Thepolymerization conditions will vary with the type of polymerizablecompound, polymerization initiator, chain transfer agent, and the like.Generally, a temperature of about 50 to 80° C., a gage pressure of about4.0 to 1.0 MPa, and a time of about 5 to 30 hours are desirable in anautoclave. Polymerization is desirably conducted in an atmosphere of agas that is inert to the reaction from the perspective of ease ofcontrolling the reaction. Examples of such gases are nitrogen and argon.In terms of cost, nitrogen is desirably employed. In the course ofpolymerization, other components that are generally added topolymerization reactions can be added to the polymerization reactionsystem in addition to the above-described components.

The weight average molecular weight of the vinyl copolymer that is thebinder of the present invention is desirably equal to or higher than1,000 from the perspective of obtaining a high-strength coating, and isdesirably equal to or lower than 200,000 from the perspective of asuitable coating material viscosity at a prescribed concentration formaintaining good working properties. From these perspectives, the weightaverage molecular weight of the vinyl copolymer that is the binder ofthe present invention is preferably 10,000 to 100,000. In the presentinvention, the “average molecular weight” refers to a value that isobtained by standard polystyrene conversion. The molecular weight of thebinder of the present invention can be controlled by means of thestarting material composition, reaction conditions, and the like.

Specific examples of the binder of the present invention are givenbelow. However, the present invention is not limited to the specificexamples given below. Mw denotes the weight average molecular weight andthe numeric values positioned to the right of the various structuralunits denote the molar ratios of the various structural units relativeto all polymerization units in the copolymers below.

The binder composition of the present invention contains the binder ofthe present invention and is desirably a composition for forming aparticulate magnetic recording medium. Specifically, it is thecomposition of the binder resin that is employed to manufacture acoating liquid for forming the powder-containing layers of a particulatemagnetic recording medium, such as a magnetic layer containing aferromagnetic powder and a nonmagnetic layer containing a nonmagneticpowder. Powder-containing layers having a high degree of surfacesmoothness can be formed by dispersing various powders to a high degreeby means of the binder of the present invention. Accordingly, the bindercomposition of the present invention, which contains the binder of thepresent invention, can permit the formation of a magnetic recordingmedium that is imparted with excellent electromagnetic characteristicsby means of high surface smoothness.

As set forth above, use of the binder of the present invention incombination with polyisocyanate can yield a magnetic recording medium ofhigh coating strength. Accordingly, the binder composition of thepresent invention desirably comprises a polyisocyanate. Examples ofpolyisocyanates are isocyanates such as trilene diisocyanate,4,4′-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylenediisocyanate, napthylene-1,5-diisocyanate, o-toluidine diisocyanate,isophorone diisocyanate, and triphenylmethane triisocyanate; products ofthese isocyanates and polyalcohols; polyisocyanates produced by thecondensation of isocyanates; and other bifunctional or greaterpolyisocyanates. Examples of product names of these isocyanates that arecurrently available are: Coronate L, Coronate HL, Coronate 2030,Coronate 2031, Millionate MR, and Millionate MTL made by NipponPolyurethane Industry Co., Ltd.; Takenate D-102, Takenate D-110N,Takenate D-200, and Takenate D-202 made by Takeda Chemical Industries,Ltd.; and Desmodule L, Desmodule IL, Desmodule N, and Desmodule HLmanufactured by Sumitomo Bayer Co., Ltd. They can be used in each layersingly or in combinations of two or more by exploiting differences incuring reactivity.

From the perspective of enhancing coating strength, it is desirable toemploy a polyisocyanate in the form of a trifunctional or greaterpolyisocyanate. Specific examples of trifunctional and greaterpolyisocyanates are adduct polyisocyanate compounds such as the compoundobtained by adding three mols of trilene diisocyanate (TDI) totrimethylol propane (TMP), the compound obtained by adding 3 mols ofhexamethylene diisocyanate (HDI) to TMP, the compound obtained by adding3 mols of isophorone diisocyanate (IPDI) to TMP, and the compoundobtained by adding xylylene diisocyanate (XDI) to TMP; condensedisocyanurate trimers of TDI; condensed isocyanurate pentamers of TDI;condensed isocyanurate heptamers of TDI; mixtures thereof; isocyanuratecondensation products of HDI; isocyanurate condensation products ofIPDI; and crude MDI. The quantity of polyisocyanate employed is, forexample, 0 to 80 weight parts per 100 weight parts of the binder of thepresent invention; from the perspective of enhancing coating strength,50 to 80 weight parts are desirable.

The binder composition of the present invention can contain variousadditives and the like that are commonly employed in coating liquids forforming magnetic recording media in addition to the binder of thepresent invention and a polyisocyanate. Magnetic powder, nonmagneticpowder, and the like can be contained in the binder composition of thepresent invention, or the composition can be mixed with these powdersfor use.

The details of the various components described above are as set forthfurther below for the magnetic recording medium of the presentinvention.

Magnetic Recording Medium

In one aspect, the magnetic recording medium of the present inventioncomprises a magnetic layer comprising a ferromagnetic powder and abinder on a nonmagnetic support, and comprises the binder of the presentinvention as the constituent component of the binder in the magneticlayer. In another aspect, the magnetic recording medium of the presentinvention comprises a nonmagnetic layer comprising a nonmagnetic powderand a binder and a magnetic layer comprising a ferromagnetic powder anda binder in this order on a nonmagnetic support, and comprises thebinder of the present invention as the constituent component of thebinder in the magnetic layer and/or in the nonmagnetic layer.

The magnetic recording medium of the present invention comprises thebinder of the present invention as a constituent component of the binderin the magnetic layer and/or nonmagnetic layer. The phrase “comprisesthe binder . . . as a constituent component” means that the binder ofthe present invention itself, or a binder in the form of a reactionproduct of the binder of the present invention and another bindercomponent, is contained. The reaction product is desirably contained asthe reaction product of the binder of the present invention (vinylcopolymer) and a polyisocyanate, as set forth above. The incorporationof such a reaction product can further increase the coating strength.

The magnetic recording medium of the present invention can contain knownthermoplastic resins, thermosetting resins, reactive resins, andmixtures thereof in addition to the binder of the present invention asconstituent components of the binder in the magnetic layer and/ornonmagnetic layer. However, high running durability that is capable ofwithstanding repeated running can be achieved without the use of a resincomponent, such as a polyurethane resin, in addition to the binder ofthe present invention (vinyl copolymer).

Examples of resin components in addition to the binder of the presentinvention are thermoplastic resins with a glass transition temperatureof −100 to 150° C. and a number average molecular weight of 1,000 to200,000, desirably 10,000 to 100,000. Specific examples are copolymerscontaining structural units in the form of polyurethane resin, vinylbutyral, vinyl acetal, vinyl ether, and the like; and variousrubber-based resins. Examples of thermosetting resins and reactiveresins are phenol resin, phenoxy resin, epoxy resin, urea resin,melamine resin, alkyd resin, formaldehyde resin, silicone resin,epoxy-polyamide resin, and mixtures of polyester resin and isocyanateprepolymers. In the magnetic recording medium of the present invention,in layers not containing the binder of the present invention, theseknown resins can be combined as desired with polyisocyanate and employedas binders. In layers containing the binder of the present inventionwith another resin and polyisocyanate, a reaction product in which thebinder of the present invention and another resin are crosslinked bypolyisocyanate can be incorporated as a constituent component of thebinder.

When employing the binder of the present invention with another resincomponent, the other resin component is desirably employed in a quantityof 1 to 100 weight parts, preferably 10 to 100 weight parts, per 100weight parts of the binder of the present invention. However, as setforth above, a magnetic recording medium of high coating strength can beobtained with the binder of the present invention even when no otherresin component is incorporated.

The binder of the present invention is desirably employed within a rangeof 5 to 50 weight parts, preferably within a range of 7 to 45 weightparts, per 100 weight parts of powder such as ferromagnetic powder andnonmagnetic powder. Use of the binder of the present invention in aquantity falling within this range relative to the various powders canenhance the dispersion of the powders. A good state of dispersion ofpowder in the magnetic recording medium can be confirmed by an increasein the surface smoothness of the magnetic recording medium. Further, thefact of a good state of dispersion of the ferromagnetic powder ornonmagnetic powder can be confirmed by exhibition of the phenomenon of ahigh degree of luster on the surface of the magnetic layer ornonmagnetic layer. Still further, use of the binder of the presentinvention in a quantity of 10 to 40 weight parts per 100 weight parts ofpowder can markedly improve electromagnetic characteristics. A contentof the binder of the present invention of equal to or greater than 5weight parts per 100 weight parts of powder is desirable in that theferromagnetic powder or nonmagnetic powder does not bind together andpowder dropout and the like do not occur. The greater the quantity ofbinder in the magnetic layer, the lower the fill rate of theferromagnetic powder and the poorer the electromagnetic characteristics.The quantity of the binder of the present invention is desirably equalto or less than 50 weight parts per 100 weight parts of ferromagneticpowder in the magnetic layer because the decrease in the fill rate ofthe ferromagnetic powder in the magnetic layer can diminish.

The magnetic recording medium of the present invention will be describedin greater detail below.

(Magnetic Layer)

In the magnetic recording medium of the present invention, theferromagnetic powder contained in the magnetic layer can be a hexagonalferrite powder. With regard to the size of the hexagonal ferrite powder,particularly when employing a magnetoresistive head in reproduction toincrease a track density, an average plate diameter equal to or lessthan 50 nm is desirable to reduce noise. An average plate diameter equalto or higher than 10 nm can yield stable magnetization without theeffects of thermal fluctuation. An average plate diameter equal to orless than 200 nm can permit low noise and is suited to the high-densitymagnetic recording. Accordingly, the average plate diameter of thehexagonal ferrite powder desirably ranges from 10 nm to 200 nm,preferably 10 nm to 50 nm. Microparticulate hexagonal ferrite powderhaving an average plate diameter within the above-stated range can behighly dispersed by means of the binder of the present invention. Theaverage plate ratio (plate diameter/plate thickness) of the hexagonalferrite powder preferably ranges from 1 to 15, more preferably from 1 to7. Low plate ratio is preferable to achieve high filling property of themagnetic layer, but sometimes adequate orientation is not achieved. Whenthe plate ratio is higher than 15, noise may be increased due tostacking between particles. The specific surface area by BET method ofthe hexagonal ferrite powders having such particle sizes normally rangesfrom 10 to 200 m²/g, almost corresponding to an arithmetic value fromthe particle plate diameter and the plate thickness. Narrowdistributions of particle plate diameter and thickness are normallygood. Although difficult to render in number form, about 500 particlescan be randomly measured in a transmission electron microscope (TEM)photograph of particles to make a comparison. This distribution is oftennot a normal distribution. However, when the distribution is expressedas the standard deviation σ to the average particle size, σ/averageparticle size=0.1 to 2.0. The particle producing reaction system isrendered as uniform as possible and the particles produced are subjectedto a distribution-enhancing treatment to achieve a narrow particle sizedistribution. For example, methods such as selectively dissolvingultrafine particles in an acid solution by dissolution are known.

A coercivity (Hc) of the hexagonal ferrite powder of about 500 to 5,000Oe (about 40 to 398 kA/m) can normally be achieved. A high coercivity(Hc) is advantageous for high-density recording, but this is limited bythe capacity of the recording head. The hexagonal ferrite powderemployed in the present invention preferably has a coercivity (Hc)ranging from 2,000 to 4,000 Oe (about 160 to 320 kA/m), more preferably2,200 to 3,500 Oe (about 176 to 280 kA/m). When the saturationmagnetization of the head exceeds 1.4 tesla, the hexagonal ferritehaving a coercivity (Hc) of equal to or higher than 2,200 Oe (aboutequal to or higher than 176 kA/m) is preferably employed. The coercivity(Hc) can be controlled by particle size (plate diameter and platethickness), the types and quantities of elements contained, substitutionsites of the element, the particle producing reaction conditions, andthe like. The saturation magnetization (σ_(s)) can be 40 to 80 A·m²/kg.The higher saturation magnetization (σ_(s)) is preferred, however, ittends to decrease with decreasing particle size. Known methods ofimproving saturation magnetization (σ_(s)) are combining spinel ferritewith magnetoplumbite ferrite, selection of the type and quantity ofelements incorporated, and the like. It is also possible to employW-type hexagonal ferrite.

For details of the hexagonal ferrite powder described above, referencecan also be made to paragraphs [0042] and [0043] of Japanese UnexaminedPatent Publication (KOKAI) No. 2004-295926, which is expresslyincorporated herein by reference in its entirety.

The ferromagnetic metal powder is also the example of the ferromagneticpowder contained in the magnetic layer. With regard to the size of theferromagnetic metal powder, the average major axis length ranging from10 nm to 100 nm is desirable, with 20 nm to 50 nm being preferable, fromthe perspective of magnetization stability and reduction of noise.Microparticulate ferromagnetic metal powder having an average major axislength within the above-stated range can be highly dispersed by means ofthe binder of the present invention.

The specific surface area (S_(BET)) by BET method of the ferromagneticmetal powder is desirably equal to or greater than 30 m²/g but less than80 m²/g, preferably equal to or greater than 40 m²/g but equal to orless than 70 m²/g, from the perspective of achieving good surfaceproperty and reduction of noise.

The coercivity (Hc) of the ferromagnetic metal powder desirably rangesfrom 1,500 to 7,000 Oe (about 119 to 557 kA/m), preferably from 2,000 to6,000 Oe (about 159 to 478 kA/m), and the as desirably ranges from 80 to170 emu/g (about 80 to 170 A·m²/kg), preferably from 90 to 160 emu/g(about 90 to 160 A·m²/kg).

The pH of the ferromagnetic metal powder is desirably optimizeddepending on the type of binder employed together. A pH range of 4 to 12can be established, with 6 to 10 being preferred. As needed, theferromagnetic metal powder can be surface treated with Al, Si, P, or anoxide thereof. The quantity of surface treatment can be set to 0.1 to 10weight percent of the ferromagnetic metal powder. When applying asurface treatment, the quantity of a lubricant such as a fatty acid thatis adsorbed is desirably not greater than 100 mg/m². The ferromagneticmetal powder will sometimes contain inorganic ions such as soluble Na,Ca, Fe, Ni, or Sr. These are desirably substantially not present in theferromagnetic metal powder, but seldom affect characteristics at 200 ppmor less. The ferromagnetic metal powder employed in the presentinvention desirably has few voids; the void level is preferably 20volume percent or less, more preferably 5 volume percent or less. Asstated above, so long as the particle size characteristics aresatisfied, the ferromagnetic metal powder may be acicular, granular,rice grain-shaped, or plate-shaped. Acicular ferromagnetic powder isdesirably employed. With regard to the acicular ferromagnetic metalpowder, the acicular ratio desirably ranges from 4 to 12, preferablyfrom 5 to 12.

For the remaining details of the ferromagnetic metal powder, referencecan also be made to paragraphs [0033] to [0035] of Japanese UnexaminedPatent Publication (KOKAI) No. 2004-295926, which is expresslyincorporated herein by reference in its entirety.

The average particle size of the ferromagnetic powder can be measured bythe following method.

Particles of ferromagnetic powder are photographed at a magnification of100,000-fold with a model H-9000 transmission electron microscope madeby Hitachi and printed on photographic paper at a total magnification of500,000-fold to obtain particle photographs. The targeted magneticmaterial is selected from the particle photographs, the contours of thepowder material are traced with a digitizer, and the size of theparticles is measured with KS-400 image analyzer software from CarlZeiss. The size of 500 particles is measured. The average value of theparticle sizes measured by the above method is adopted as an averageparticle size of the ferromagnetic powder.

The size of a powder such as the magnetic material (referred to as the“powder size” hereinafter) in the present invention is denoted: (1) bythe length of the major axis constituting the powder, that is, the majoraxis length, when the powder is acicular, spindle-shaped, or columnar inshape (and the height is greater than the maximum major diameter of thebottom surface); (2) by the maximum major diameter of the tabularsurface or bottom surface when the powder is tabular or columnar inshape (and the thickness or height is smaller than the maximum majordiameter of the tabular surface or bottom surface); and (3) by thediameter of an equivalent circle when the powder is spherical,polyhedral, or of unspecified shape and the major axis constituting thepowder cannot be specified based on shape. The “diameter of anequivalent circle” refers to that obtained by the circular projectionmethod.

The average powder size of the powder is the arithmetic average of theabove powder size and is calculated by measuring five hundred primaryparticles in the above-described method. The term “primary particle”refers to a nonaggregated, independent particle.

The average acicular ratio of the powder refers to the arithmeticaverage of the value of the (major axis length/minor axis length) ofeach powder, obtained by measuring the length of the minor axis of thepowder in the above measurement, that is, the minor axis length. Theterm “minor axis length” means the length of the minor axis constitutinga powder for a powder size of definition (1) above, and refers to thethickness or height for definition (2) above. For (3) above, the (majoraxis length/minor axis length) can be deemed for the sake of convenienceto be 1, since there is no difference between the major and minor axes.

When the shape of the powder is specified, for example, as in powdersize definition (1) above, the average powder size refers to the averagemajor axis length. For definition (2) above, the average powder sizerefers to the average plate diameter, with the arithmetic average of(maximum major diameter/thickness or height) being referred to as theaverage plate ratio. For definition (3), the average powder size refersto the average diameter (also called the average particle diameter).

(Nonmagnetic Layer)

In one aspect of the magnetic recording medium of the present invention,there is a nonmagnetic layer comprising a nonmagnetic powder and abinder between the nonmagnetic support and magnetic layer, and thebinder in the magnetic layer and/or nonmagnetic layer contains thebinder of the present invention as a constituent component. The binderof the present invention can contribute to enhancing the dispersibilityof the nonmagnetic powder and to increasing the coating strength in thenonmagnetic layer in the same manner as in the magnetic layer.

Both organic and inorganic substances may be employed as the nonmagneticpowder in the nonmagnetic layer. Carbon black may also be employed.Examples of inorganic substances are metals, metal oxides, metalcarbonates, metal sulfates, metal nitrides, metal carbides, and metalsulfides. Specifically, titanium oxides such as titanium dioxide, ceriumoxide, tin oxide, tungsten oxide, ZnO, ZrO₂, SiO₂, Cr₂O₃, α-alumina withan α-conversion rate of 90 to 100 percent, β-alumina, γ-alumina, α-ironoxide, goethite, corundum, silicon nitride, titanium carbide, magnesiumoxide, boron nitride, molybdenum disulfide, copper oxide, MgCO₃, CaCO₃,BaCO₃, SrCO₃, BaSO₄, silicon carbide, and titanium carbide may beemployed singly or in combinations of two or more. α-iron oxide andtitanium oxide are preferred.

The nonmagnetic powder may be acicular, spherical, polyhedral, orplate-shaped. The average particle diameter of the nonmagnetic powderdesirably ranges from 0.005 μm to 2 μm, preferably from 0.01 μm to 0.2μm. As needed, nonmagnetic powders of differing average particlediameter may be combined; the same effect may be achieved by broadeningthe average particle distribution of a single nonmagnetic powder. Thecrystallite size of the nonmagnetic powder desirably ranges from 0.004μm to 1 μm, preferably from 0.04 μm to 0.1 μm. The specific surface areaof the nonmagnetic powder desirably ranges from 1 to 100 m²/g,preferably from 5 to 70 m²/g, and more preferably from 10 to 65 m²/g.Microparticulate nonmagnetic powder having the above-stated size can behighly dispersed by means of the binder of the present invention.

The oil absorption capacity using dibutyl phthalate (DBP) of thenonmagnetic powder is desirably 5 to 100 mL/100 g, preferably 10 to 80mL/100 g, and more preferably, 20 to 60 mL/100 g. The specific gravityis desirably 1 to 12, preferably 3 to 6. The tap density is desirably0.05 to 2 g/mL, preferably 0.2 to 1.5 g/mL. At less than 0.05 g/mL,there are numerous scattering particles and handling tends to becomedifficult. At greater than 2 g/mL, there tends to be adhesion to thedevice and handling tends to become difficult. The pH of the nonmagneticpowder is desirably 2 to 11, preferably 6 to 9. When the pH falls withinthis range, an increase in the coefficient of friction can besuppressed.

The presence of Al₂O₃, SiO₂, TiO₂, ZrO₂, SnO₂, Sb₂O₃, and ZnO isdesirable through the application of surface treatments to the surfaceof the nonmagnetic powders. Al₂O₃, SiO₂, TiO₂, and ZrO₂ are preferablefor dispersibility, and Al₂O₃, SiO₂, and ZrO₂ are of still greaterpreference. They can be combined for use, or employed singly. Dependingon the objective, a surface-treatment coating layer with acoprecipitated material may also be employed, the method which comprisesa first alumina coating and a second silica coating thereover or thereverse method thereof may also be adopted. Depending on the objective,the surface-treatment coating layer may be a porous layer, withhomogeneity and density being generally desirable.

For the remaining details regarding the nonmagnetic powder, referencecan be made to paragraphs [0047] to [0048] and [0050] in JapaneseUnexamined Patent Publication (KOKAI) No. 2004-295926, which isexpressly incorporated herein by reference in its entirety.

(Additives)

Additives can be added as needed to the magnetic layer and nonmagneticlayer. Additives in the form of compounds that impart dispersingeffects, leveling effects, antistatic effects, plasticizing effects, andthe like can be employed. Reference can be made to paragraphs [0055] to[0060] of Japanese Unexamined Patent Publication (KOKAI) No. 2004-295926for details regarding compounds that can be employed as additives, whichis expressly incorporated herein by reference in its entirety.

Carbon black may be added to the magnetic layer and/or nonmagnetic layeras needed. Examples of types of carbon black that are suitable for useare: furnace black for rubber, thermal for rubber, black for coloring,and acetylene black. The specific surface area is desirably 5 to 500m²/g, the DBP oil absorption capacity is desirably 10 to 400 ml/100 g,the average particle diameter is desirably 5 to 300 nm, preferably 10 to250 nm, and more preferably, 20 to 200 nm. The pH is desirably 2 to 10,the moisture content is desirably 0.1 to 10 percent, and the tap densityis desirably 0.1 to 1 g/cc. When employing carbon black, the quantitydesirably ranges from 0.1 to 30 weight percent with respect to theweight of the ferromagnetic powder or the nonmagnetic powder. Forexample, the Carbon Black Handbook compiled by the Carbon BlackAssociation, which is expressly incorporated herein by reference in itsentirety, may be consulted for types of carbon black suitable for use inthe present invention.

(Organic Solvent)

Known organic solvents can be used. Examples are ketones such asacetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone,cyclohexanone, isophorone, and tetrahydrofuran; alcohols such asmethanol, ethanol, propanol, butanol, isobutyl alcohol, isopropylalcohol, and methylcyclohexanol; esters such as methyl acetate, butylacetate, isobutyl acetate, isopropyl acetate, ethyl lactate, and glycolacetate; glycol ethers such as glycol dimethyl ether, glycol monoethylether, and dioxane; aromatic hydrocarbons such as benzene, toluene,xylene, cresol, and chlorobenzene; chlorinated hydrocarbons such asmethylene chloride, ethylene chloride, carbon tetrachloride, chloroform,ethylene chlorohydrin, and dichlorobenzene; N,N-dimethylformamide; andhexane; these may be employed in any ratio.

All or some part of the additives employed in the present invention canbe added in any of the steps during the manufacturing of coating liquidsfor the magnetic layer and nonmagnetic layer. For example, there arecases where they are mixed with the ferromagnetic powder prior to thekneading step; cases where they are added during the step in which theferromagnetic powder, binder, and solvent are kneaded; cases where theyare added during the dispersion step; cases where they are added afterdispersion; and cases where they are added directly before coating.

(Nonmagnetic Support)

Known films of the following may be employed as the nonmagnetic support:biaxially-oriented polyethylene naphthalate, polyethylene terephthalate,polyamides, polyimides, polyamidoimides, aromatic polyamides,polybenzooxazoles, and the like, with polyethylene naphthalate andaromatic polyamides being preferred. These supports may be subjectedbeforehand to corona discharge treatment, plasma treatment, adhesionenhancing treatment, heat treatment, and the like.

The nonmagnetic support that can be employed in the present inventiondesirably has good smoothness in the form of a centerline averagesurface roughness of 0.1 to 20 nm, preferably 1 to 10 nm at a cut-offvalue of 0.25 nm. These nonmagnetic supports not only are of lowcenterline average surface roughness, but also desirably have no coarseprotrusions of 1 micrometer or greater. The arithmetic average roughnessof the support obtained is desirably equal to or lower than 0.1 μm asthe Ra specified in JIS B0660-1998 and ISO 4287-1997 to reduce noise.

(Layer Structure)

In the magnetic recording medium of the present invention, the thicknessof the nonmagnetic support is, for example, 2 to 100 μm, desirably 3 to80 μm. An undercoating layer for increasing adhesion or a smoothinglayer to increase smoothness can be provided between the nonmagneticsupport and the nonmagnetic layer or magnetic layer. The thickness ofthe undercoating layer and smoothing layer is, for example, 0.01 to 0.5μm, desirably 0.02 to 0.5 μm. The magnetic recording medium of thepresent invention can be a disk-like medium with a nonmagnetic layer andmagnetic layer provided on both sides of the support, or a tape-likemedium or disk-like medium in which they are provided on just one side.In that case, a backcoat layer can be provided on the opposite side fromthe side on which the nonmagnetic layer and magnetic layer are providedso as to achieve antistatic, curling correction, and like effects. Thethickness of the backcoat layer is, for example, 0.1 to 4 μm, desirably0.3 to 2.0 μm. Known undercoating layers, smoothing layers, and backcoatlayers can be employed. For the details, reference can be made toparagraphs [0064] to [0066] in Japanese Unexamined Patent Publication(KOKAI) No. 2004-295926, which is expressly incorporated herein byreference in its entirety. The binder of the present invention can alsobe incorporated into these layers in the magnetic recording medium ofthe present invention.

The thickness of the nonmagnetic layer is normally 0.2 to 5.0 μm,desirably 0.3 to 3.0 μm, and preferably, 0.4 to 2.0 μm.

The thickness of the magnetic layer is desirably 0.01 to 0.10 μm,preferably 0.02 to 0.08 μm, and more preferably, 0.03 to 0.08 μm. It isdesirably optimized based on the saturation magnetization and head gaplength of the magnetic head employed, and on the band of the recordingsignal. A single magnetic layer suffices, but the magnetic layer can bedivided into two or more layers of differing magnetic characteristics. Aknown multilayer magnetic layer configuration can be employed.Generally, the thinner the magnetic layer, the lower the coatingdurability, and the more difficult it is to maintain good runningdurability. In contrast, incorporating the binder of the presentinvention into the magnetic layer can increase the coating strength ofthe magnetic layer. Thus, good running durability can be achieved in amagnetic recording medium having a thin magnetic layer with a thicknessfalling within the above range.

(Preparation of Coating Liquid)

The process for manufacturing coating liquids for each layer such as themagnetic layer, nonmagnetic layer and backcoat layer can comprise atleast a kneading step, a dispersing step, and a mixing step to becarried out, if necessary, before and/or after the kneading anddispersing steps. Each of the individual steps may be divided into twoor more stages. All of the starting materials employed in the presentinvention, including the ferromagnetic powder, nonmagnetic powder,binders, carbon black, abrasives, antistatic agents, lubricants,solvents, and the like, may be added at the beginning of, or during, anyof the steps. Moreover, the individual starting materials may be dividedup and added during two or more steps. To achieve the object of thepresent invention, conventionally known manufacturing techniques may beutilized for some of the steps. Further, glass beads may be employed todisperse the coating liquids for each layer, with a dispersing mediumwith a high specific gravity such as zirconia beads, titania beads, andsteel beads being suitable for use. The particle diameter and fill ratioof these dispersing media can be optimized for use. A known dispersingdevice may be employed.

In the method of manufacturing a magnetic recording medium, for example,a magnetic layer can be formed by coating a magnetic layer coatingliquid to a prescribed thickness on the surface of a nonmagnetic supportthat is being run. Multiple magnetic layer coating liquids can besuccessively or simultaneously coated in a multilayer coating, or anonmagnetic layer coating liquid and a magnetic layer coating liquid canbe successively or simultaneously coated in a multilayer coating.Coating machines suitable for use in coating the coating liquid for eachlayer are air doctor coaters, blade coaters, rod coaters, extrusioncoaters, air knife coaters, squeeze coaters, immersion coaters, reverseroll coaters, transfer roll coaters, gravure coaters, kiss coaters, castcoaters, spray coaters, spin coaters, and the like. For example, “RecentCoating Techniques” (May 31, 1983), issued by the Sogo Gijutsu CenterK.K. may be referred to in this regard. The content of the abovepublication is expressly incorporated herein by reference in itsentirety. For the details regarding the coating process, reference canalso be made to paragraphs [0067] and [0068] of Japanese UnexaminedPatent Publication (KOKAI) No. 2004-295926, which is expresslyincorporated herein by reference in its entirety.

The medium after the coating process can be subjected to post-processingsuch as orientation processing of the magnetic layer, surface smoothingprocessing (calendering), and the like. For the details regarding thepost-processing, reference can also be made to paragraphs [0070] to[0073] of Japanese Unexamined Patent Publication (KOKAI) No.2004-295926, which is expressly incorporated herein by reference in itsentirety. The magnetic recording medium that is obtained can be cut todesired size with a cutter, punching machine the like for use.

EXAMPLES

The present invention will be described in detail below based onExamples. However, the present invention is not limited to the examples.The “parts” given in Examples are weight parts unless specificallystated otherwise. The weight average molecular weights described beloware values that were obtained by conversion to standard polystyreneusing DMF solvent containing 0.3 weight percent lithium bromide. As forthe copolymers containing multiple structural units, the numeric valuespositioned to the right of the various structural units denote the molarratios of the various structural units relative to all polymerizationunits in the copolymers below.

Reference Example 1 Synthesis of a Graft Chain Monomer (Example CompoundG-6)

To a reaction vessel equipped with stirrer and reflux condenser werecharged 100 g of methyl methacrylate, 4.61 g of thioglycolic acid, 0.164g of AIBN, and 10 g of THF, the mixture was heated at 80° C. for onehour in a nitrogen atmosphere, and the reaction was ended. Purificationby reprecipitation from 5 L of hexane was conducted and the mixture wasdried for 3 hours 50° C. in a vacuum dryer, yielding a white solid. Thenumber average molecular weight was 2.4×10³.

To a reaction vessel equipped with stirrer and reflux condenser werecharged 10 g of the above white solid, 1.35 g of glycidyl methacrylate,0.01 g of hydroquinone, 0.01 g of N,N-dimethyllaurylamine, and 20 g ofxylene, the mixture was hot refluxed for five hours in a nitrogenatmosphere, and the reaction was ended. Following the reaction,purification by reprecipitation was conducted by the same method asabove, yielding G-6. The number average molecular weight was 2.3×10³.

The graft chain monomers employed in the synthesis examples below weresynthesized by the same method as in Reference Example 1 by varying thetype and weight ratio (molar percentages) of the monomers employed.

Synthesis Example 1 Synthesis of Vinyl Copolymer Having Structural Units[G] and [P]

To a reaction vessel equipped with stirrer and reflux condenser werecharged 17.1 g of Example Compound G-2, 8.5 g of methyl methacrylate,0.13 g of methacrylic acid, 0.27 g of dimethyl 2,2′-azobisisobutyrate,and 52.0 g of 2-butanone, the mixture was heated to 78° C. for 9 hoursin a nitrogen atmosphere, and the reaction was ended. The weight averagemolecular weight was the above value. Following the reaction, GPCanalysis revealed no residual monomer or residual oligomer peaks. Thus,the fact that a copolymer incorporating the various structural units inthe charge ratio had been obtained was confirmed.

Synthesis Examples 2 to 9 Synthesis of Acrylic Copolymers HavingStructural Units [G], [P], [1], [2], and [3]

Acrylic copolymers P-4, P-6, P-9, P-11, P-16, P-20, P-23, and P-25 wereobtained by the same method as in Synthesis Example 1 by varying thetypes and weight ratios (molar percentages) of the monomers employed.The weight average molecular weight of each of the copolymers wasmeasured, and the values indicated above were confirmed. Following thereaction, GPC analysis revealed no residual monomer or residual oligomerpeaks. Thus, it was confirmed that copolymers incorporating the variousstructural units in the charge ratio had been obtained.

The content of the polar groups (polar groups selected from the groupconsisting of sulfonic acid (salt) groups, carboxylic acid (salt)groups, and phosphoric acid (salt) groups) of the copolymers obtained inthe synthesis examples were calculated from the charge quantities of thestarting material monomers. The results are given below.

-   Synthesis Example 1 (Example Compound P-2): 60 eq/g-   Synthesis Example 2 (Example Compound P-4): 33 eq/g-   Synthesis Example 3 (Example Compound P-6): 83 eq/g-   Synthesis Example 4 (Example Compound P-9): 33 eq/g-   Synthesis Example 5 (Example Compound P-11): 56 eq/g-   Synthesis Example 6 (Example Compound P-16): 146 eq/g-   Synthesis Example 7 (Example Compound P-20): 39 eq/g-   Synthesis Example 8 (Example Compound P-23): 74 eq/g-   Synthesis Example 9 (Example Compound P-25): 60 eq/g

Comparative Synthesis Example 1 Synthesis of Acrylic Copolymer BP-1Comprising Just Structural Unit [G]

To a reaction vessel equipped with stirrer and reflux condenser werecharged 25.6 g of Example Compound G-1, 0.1 g of dimethyl2,2′-azobisisobutyrate, and 31.0 g of 2-butanone. The mixture was heatedto 78° C. for nine hours in a nitrogen atmosphere and the reaction wasended, yielding acrylic copolymer BP-1 having the above structural unit.The weight average molecular weight was 6.0×10⁵.

Comparative Synthesis Example 2 Synthesis of Acrylic Copolymer BP-2Comprising Just Structural Unit [P]

To a reaction vessel equipped with stirrer and reflux condenser werecharged 22.0 g of methacrylic acid, 1.0 g of dimethyl2,2′-azobisisobutyrate, 1.6 g of dimethyl 2,2′-azobisisobutyrate, and31.0 g of N-methylpyrrolidinone. The mixture was heated to 78° C. fornine hours in a nitrogen atmosphere and the reaction was ended, yieldingacrylic copolymer BP-2 having the above structural unit. The weightaverage molecular weight was 1.5×10⁶.

Comparative Synthesis Example 3 Synthesis of Acrylic Copolymer BP-3Comprising Structural Units [P], [1], [2], and [3]

To a reaction vessel equipped with stirrer and reflux condenser werecharged 37.1 g of Example Compound A-1, 14.2 g of stearyl methacrylate,27.9 g of 2,2-hydroxyethyl acrylate, 0.24 g of AMPS, and 74.5 g ofN-methylpyrrolidinone. The mixture was heated to 78° C. for nine hoursin a nitrogen atmosphere and the reaction was ended, yielding acryliccopolymer BP-3 having the above structural units.

The weight average molecular weight was 7.0×10⁵. Following the reaction,GPC analysis revealed no residual monomer or residual oligomer peaks.Thus, it was confirmed that copolymers incorporating the variousstructural units in the charge ratio had been obtained.

Example 1-1

(1) Preparation of Magnetic Layer Coating Liquid

Ferromagnetic metal powder: 100 parts

-   -   Composition: Fe/Co=100/25    -   Hc: 2,450 Oe (approx. 195 kA/m)    -   Specific surface area by BET method: 65 m²/g    -   Surface treatment agents: Al₂O₃, SiO₂, Y₂O₃    -   Particle size (average major axis length): 45 nm    -   Acicular ratio: 5    -   σs: 110 emu/g (approx. 110 A·m²/kg)

Phenylphosphonic acid: 3 parts

Acrylic copolymer P-2: 15 parts

Methyl ethyl ketone: 150 parts

Cyclohexanone: 150 parts

α-Al₂O₃ Mohs' hardness 9 (average particle diameter: 0.1 μm): 15 parts

Carbon black (average particle size: 0.08 μm): 0.5 part

The above components were kneaded in an open kneader and dispersed usinga ball mill. The following components were added to the dispersionobtained. The mixture was stirred, ultrasonically processed, and passedthrough a filter having an average pore diameter of 1 μm to prepare amagnetic layer coating liquid.

Butyl stearate: 1.5 parts

Stearic acid: 0.5 part

Methyl ethyl ketone: 50 parts

Cyclohexanone: 50 parts

Toluene: 3 parts

Polyisocyanate compound (Coronate 3041 made by Nippon PolyurethaneIndustry Co., Ltd.): 5 parts

(2) Preparation of Nonmagnetic Layer Coating Liquid

Nonmagnetic powder (α-Fe₂O₃ hematite): 80 parts

-   -   Average major axis length: 0.15 μm    -   Specific surface area by BET method: 52 m²/g    -   pH: 6    -   Tap density: 0.8    -   DBP oil absorption capacity: 27 to 38 g/100 g    -   Surface treatment agents: Al₂O₃, SiO₂

Carbon black: 20 parts

-   -   Average primary particle diameter: 0.020 μm    -   DBP oil absorption capacity: 80 mL/100 g    -   pH: 8.0    -   Specific surface area by BET method: 250 m²/g    -   Volatile content: 1.5 percent

Acrylic copolymer P-2: 19 parts

Methyl ethyl ketone: 150 parts

Cyclohexanone: 150 parts

The above components were kneaded in an open kneader and dispersed usinga ball mill. The following components were added to the dispersionobtained. The mixture was stirred, ultrasonically processed, and passedthrough a filter having an average pore diameter of 1 μm to prepare alower coating layer (nonmagnetic layer) coating liquid.

Butyl stearate: 1.5 parts

Stearic acid: 1 part

Methyl ethyl ketone: 50 parts

Cyclohexanone: 50 parts

Toluene: 3 parts

Polyisocyanate compound (Coronate 3041 made by Nippon PolyurethaneIndustry Co., Ltd.): 5 parts

(3) Preparation of Backcoat Layer Coating Liquid

Carbon black (average particle diameter 40 nm): 85 parts

Carbon black (average particle diameter 100 nm): 3 parts

Nitrocellulose: 28 parts

Polyurethane resin: 58 parts

Copper phthalocyanine dispersing agent: 2.5 parts

Nipporan 2301 (made by Nippon Polyurethane Industry Co., Ltd.): 0.5 part

Methyl isobutyl ketone: 0.3 part

Methyl ethyl ketone: 860 parts

Toluene: 240 parts

The above components were prekneaded in a roll mill and dispersed in asand mill. Four parts of polyester resin (Vylon 500 made by Toyobo Co.,Ltd.), 14 parts of polyisocyanate compound (Coronate 3041 made by NipponPolyurethane Industry Co., Ltd.), and 5 parts of α-Al₂O₃ (made bySumitomo Chemicals) were added and the mixture was stirred and filteredto prepare a backcoat layer coating liquid.

(4) Preparation of Magnetic Tape

Simultaneous multilayer coating was conducted by applying the abovenonmagnetic layer coating liquid in a quantity calculated to yield a drythickness of 1.0 μm on a polyethylene naphthalate resin support that hadbeen corona processed in advance to render the base surface hydrophilic,was 5 μm in thickness, and had a centerline surface roughness of themagnetic layer coating surface of 0.001 μm, and immediately thereafter,applying a magnetic layer to a thickness of 0.1 μm thereover. While thetwo layers were still wet, orientation was conducted with a cobaltmagnetic having a magnetic force of 0.5 T (5,000 G) and a solenoidhaving a magnetic force of 0.4 T (4,000 G), and then dried.Subsequently, the above backcoat layer coating liquid was applied in aquantity calculated to yield a dry thickness of 0.5 μm on the oppositesurface from the above-described base surface, which had also beencorona processed in advance. The product was then processed at a rate of80 m/min at a temperature of 100° C. with a seven-stage calendercomprised of metal rolls and slit to a width of ½ mm to prepare amagnetic tape.

Examples 1-2 to 1-9

With the exception that the types of acrylic copolymer employed in themagnetic layer and nonmagnetic layer were changed as indicated in Table1, the magnetic tapes of Examples 1-2 to 1-9 were prepared in the samemanner as in Example 1-1.

Comparative Example 1-1

With the exception that the types of acrylic copolymer employed in themagnetic layer and nonmagnetic layer were changed to those described inExample 2 of Japanese Unexamined Patent Publication (KOKAI) Heisei No.8-67855, the magnetic tape of Comparative Example 1-1 was prepared inthe same manner as in Example 1-1.

Comparative Example 1-2

With the exception that the types of acrylic copolymer employed in themagnetic layer and nonmagnetic layer were changed to those described inExample 3 of Japanese Unexamined Patent Publication (KOKAI) No.2004-295926, the magnetic tape of Comparative Example 1-2 was preparedin the same manner as in Example 1-1.

Comparative Example 1-3

With the exception that the types of acrylic copolymer employed in themagnetic layer and nonmagnetic layer were changed to those described inExample 4 of Japanese Unexamined Patent Publication (KOKAI) Heisei No.6-111277, the magnetic tape of Comparative Example 1-3 was prepared inthe same manner as in Example 1-1.

Comparative Example 1-4

With the exception that the types of acrylic copolymer employed in themagnetic layer and nonmagnetic layer were changed to those described inReference Example 1 of Japanese Examined Patent Publication (KOKAI)Heisei No. 7-9696, the magnetic tape of Comparative Example 1-4 wasprepared in the same manner as in Example 1-1.

Comparative Example 1-5

With the exception that the types of acrylic copolymer employed in themagnetic layer and nonmagnetic layer were changed to those described inReference Example 7 of Japanese Examined Patent Publication (KOKAI)Heisei No. 7-9696, the magnetic tape of Comparative Example 1-5 wasprepared in the same manner as in Example 1-1.

Comparative Examples 1-6 to 1-8

With the exception that the types of acrylic copolymer employed in themagnetic layer and nonmagnetic layer were changed to those indicated inTable 1, the magnetic tapes of Comparative Examples 1-6 to 1-8 wereprepared in the same manner as in Example 1-1.

Example 2-1

(1) Preparation of Ferromagnetic Hexagonal Ferrite Magnetic LayerCoating Liquid

Ferromagnetic plate-like hexagonal ferrite powder: 100 parts

-   -   Composition excluding oxygen (molar ratio):        Ba/Fe/Co/Zn=1/9/0.2/1    -   Hc: 160 kA/m (2,000 Oe)    -   Average plate diameter: 20 nm    -   Average plate ratio: 2.7    -   BET specific surface area: 60 m²/g    -   σs: 46 A·m²/kg (46 emu/g)

Acrylic copolymer P-2: 12 parts

α-Al₂O₃ (particle size: 0.1 μm): 8 parts

Carbon black (average particle diameter: 20 nm): 0.5 part

Cyclohexanone: 110 parts

The above components were kneaded in an open kneader and dispersed in asand mill. The following components were added to the dispersionobtained. The mixture was stirred, ultrasonically processed, and passedthrough a filter having an average pore diameter of 1 μm to prepare amagnetic layer coating liquid.

Butyl stearate: 2.0 parts

Stearic acid: 0.5 part

Methyl ethyl ketone: 50 parts

Cyclohexanone: 50 parts

Toluene: 3 parts

Polyisocyanate compound (Coronate 3041 made by Nippon PolyurethaneIndustry Co., Ltd.): 5 parts

Simultaneous multilayer coating was conducted by applying a nonmagneticlayer coating liquid prepared by the same method as in Example 1-1 in aquantity calculated to yield a dry thickness of 1.0 μm on a polyethylenenaphthalate resin support that had been corona processed in advance torender the base surface hydrophilic, was 5 μm in thickness, and had acenterline surface roughness of the magnetic layer coating surface of0.001 μm, and immediately thereafter, applying thereover the abovemagnetic layer coating liquid in a quantity calculated to yield amagnetic layer 0.1 μm in thickness. While the two layers were still wet,orientation was conducted with a cobalt magnet having a magnetic forceof 0.5 T (5,000 G) and a solenoid having a magnetic force of 0.4 T(4,000 G). Subsequently, a backcoat layer coating liquid that had beenprepared by the same method as in Example 1-1 was applied in a quantitycalculated to yield a dry thickness of 0.5 μm on the opposite surfacefrom the above-described base surface, which had also been coronaprocessed in advance. The product was then processed at a rate of 80m/min at a temperature of 100° C. with a seven-stage calender comprisedof metal rolls and slit to a width of ½ mm to prepare the magnetic tapeof Example 2-1.

Examples 2-2 to 2-9

With the exception that the types of acrylic copolymer employed in themagnetic layer and nonmagnetic layer were changed to those indicated inTable 2, the magnetic tapes of Examples 2-2 to 2-9 were prepared in thesame manner as in Example 2-1.

Comparative Example 2-1

With the exception that the types of acrylic copolymer employed in themagnetic layer and nonmagnetic layer were changed to those described inExample 2 of Japanese Unexamined Patent Publication (KOKAI) Heisei No.8-67855, the magnetic tape of Comparative Example 2-1 was prepared inthe same manner as in Example 2-1.

Comparative Example 2-2

With the exception that the types of acrylic copolymer employed in themagnetic layer and nonmagnetic layer were changed to those described inExample 3 of Japanese Unexamined Patent Publication (KOKAI) No.2004-295926, the magnetic tape of Comparative Example 2-2 was preparedin the same manner as in Example 2-1.

Comparative Example 2-3

With the exception that the types of acrylic copolymer employed in themagnetic layer and nonmagnetic layer were changed to those described inExample 4 of Japanese Unexamined Patent Publication (KOKAI) Heisei No.6-111277, the magnetic tape of Comparative Example 2-3 was prepared inthe same manner as in Example 2-1.

Comparative Example 2-4

With the exception that the types of acrylic copolymer employed in themagnetic layer and nonmagnetic layer were changed to those described inReference Example 1 of Japanese Examined Patent Publication (KOKAI)Heisei No. 7-9696, the magnetic tape of Comparative Example 2-4 wasprepared in the same manner as in Example 2-1.

Comparative Example 2-5

With the exception that the types of acrylic copolymer employed in themagnetic layer and nonmagnetic layer were changed to those described inReference Example 7 of Japanese Examined Patent Publication (KOKAI)Heisei No. 7-9696, the magnetic tape of Comparative Example 2-5 wasprepared in the same manner as in Example 2-1.

Comparative Examples 2-6 to 2-8

With the exception that the types of acrylic copolymer employed in themagnetic layers and nonmagnetic layers were changed to those indicatedin Table 2, the magnetic tapes of Comparative Examples 2-6 to 2-8 wereprepared in the same manner as in Example 2-1.

Measurement Methods Average Surface Roughness of Tape

A 40×40 micrometer area of the magnetic layer surface was measured incontact mode with an atomic force microscope (AFM: Nanoscope III made byDigital Instruments) and the centerline average surface roughness (Ra)was measured.

Electromagnetic Characteristics: S/N Ratio

Signals were recorded at linear recording densities of 172 kfci and 86kfci on recording tracks of 11.5 μm with a reproduction track width of5.3 μm using an LTO-Gen4 drive. The reproduced signal was frequencyanalyzed with a spectrum analyzer. The ratio of the output of thecarrier signal during 172 kfci signal recording to the noise integratedover the entire spectral band during 86 kfci signal recording wasadopted as the S/N ratio. A FUJIFILM LTO-Gen4 tape was employed as areference tape. The S/N ratio of the reference tape was adopted as 0 dBand the S/N values of the various tapes were calculated as relativevalues. An S/N ratio of equal to or greater than 0 dB indicated goodelectromagnetic characteristics as a magnetic recording medium forhigh-density recording.

Amount of Grime on Tape Surface

The tape was run at an angle of 150° so that the magnetic layer surfacecontacted the edge of a square bar having a cross section of 7×7 mm thatwas made of Al₂O₃/TiC. Under conditions of a load of 100 g and a rate of6 m/s, a 100 m length was slid during each pass. The edge of the squarebar was then observed under a microscope and the state of adhesion ofgrime was evaluated. Sensory evaluation was conducted on a scale of 1 to10. A rating of 10 indicated little grime, and a rating of 1 indicatedmaximum grime.

The grime evaluated by the above method was primarily produced byshaving of the magnetic layer surface. The lower the value of theevaluation result, the greater the shaving of the magnetic layer surfaceand the poorer the running durability.

TABLE 1 Examples and Comparative Examples using ferromagnetic metalpowder Dispersibility Running Surface durability property Amount ofgrime Acrylic copolymer Ra(nm) S/N(dB) (Poor)1-10(Good) Ex. 1-1 P-2 2.21.0 5 Ex. 1-2 P-4 2.3 1.0 5 Ex. 1-3 P-6 2.4 1.5 5 Ex. 1-4 P-9 2.3 1.0 7Ex. 1-5 P-11 2.0 2.5 9 Ex. 1-6 P-16 1.9 2.5 8 Ex. 1-7 P-20 2.0 2.0 8 Ex.1-8 P-23 1.9 2.0 8 Ex. 1-9 P-25 1.8 3.0 10 Comp. Ex. 1-1 Example 2 inJapanese Unexamined 5.0 −5.0 3 Patent Publication (KOKAI) Heisei No.8-67855 Comp. Ex. 1-2 Example 3 of Japanese Unexamined 4.7 −3.0 4 PatentPublication (KOKAI) No. 2004-295926 Comp. Ex. 1-3 Example 4 in JapaneseUnexamined 4.5 −3.5 7 Patent Publication (KOKAI) Heisei No. 6-111277Comp. Ex. 1-4 Reference Example 1 3.0 −1.0 1 in Japanese Examined PatentPublication (KOKAI) Heisei No. 7-9696 Comp. Ex. 1-5 Reference Example 73.5 −1.5 2 in Japanese Examined Patent Publication (KOKAI) Heisei No.7-9696 Comp. Ex. 1-6 BP-1 4.0 −2.0 3 Comp. Ex. 1-7 BP-2 6.9 −5.0 1 Comp.Ex. 1-8 BP-3 2.8 0.0 7

TABLE 2 Examples and Comparative Examples using hexagonal ferrite powderDispersibility Surface Running durability property Amount of grimeAcrylic copolymer Ra(nm) S/N(dB) (Poor)1-10(Good) Ex. 2-1 P-2 2.0 1.0 5Ex. 2-2 P-4 2.1 1.0 5 Ex. 2-3 P-6 2.0 1.5 6 Ex. 2-4 P-9 2.1 2.0 7 Ex.2-5 P-11 1.8 3.5 10 Ex. 2-6 P-16 1.7 3.0 10 Ex. 2-7 P-20 1.8 3.5 10 Ex.2-8 P-23 1.9 3.0 10 Ex. 2-9 P-25 1.7 3.0 10 Comp. Ex. 2-1 Example 2 inJapanese Unexamined 3.1 −2.0 10 Patent Publication (KOKAI) Heisei No.8-67855 Comp. Ex. 2-2 Example 3 of Japanese Unexamined 4.0 −2.0 4 PatentPublication (KOKAI) No. 2004-295926 Comp. Ex. 2-3 Example 4 in JapaneseUnexamined 3.5 −1.5 7 Patent Publication (KOKAI) Heisei No. 6-111277Comp. Ex. 2-4 Reference Example 1 3.0 0.0 2 in Japanese Examined PatentPublication (KOKAI) Heisei No. 7-9696 Comp. Ex. 2-5 Reference Example 73.1 −1.0 1 in Japanese Examined Patent Publication (KOKAI) Heisei No.7-9696 Comp. Ex. 2-6 BP-1 3.1 −1.0 3 Comp. Ex. 2-7 BP-2 5.5 −1.5 1 Comp.Ex. 2-8 BP-3 2.9 0.0 8Evaluation Results

The magnetic tapes of the Examples formed using the binder of thepresent invention had highly smooth surfaces. Thus, the fact that themicroparticulate powder was dispersed to a high degree by the binder ofthe present invention was confirmed. As a result, the magnetic tapes ofthe Examples exhibited good electromagnetic characteristics.

-   (2) By contrast, the magnetic tapes of Comparative Examples 1-1 to    1-5 and 2-1 to 2-5, in which the vinyl copolymers conventional    employed as binders in magnetic recording media were employed,    exhibited poorer results than the magnetic tapes of the Examples in    the evaluation items of surface smoothness and electromagnetic    characteristics.-   (3) The magnetic tapes of Comparative Examples 1-6 and 2-6, in which    copolymers having only structural unit [G] was employed, and the    magnetic tapes of Comparative Examples 1-7 and 2-7, in which    copolymers having only structural unit [P] was employed, exhibited    poorer results than the magnetic tapes of the Examples in the    evaluation items of surface smoothness and electromagnetic    characteristics. Further, the magnetic tapes of Comparative Examples    1-8 and 2-8, in which copolymers containing structural units [P],    [1], [2], and [3] but not structural unit [G] were employed,    exhibited poorer results than the Examples despite relatively    enhanced surface smoothness and electromagnetic characteristics.    Thus, the combination of structural units [G] and [P] was determined    to make it possible to achieve both electromagnetic characteristics    and running durability.-   (4) Further improvement in surface smoothness and good running    durability was confirmed in magnetic tapes containing binder in the    form of copolymers having structural units [1], [2], and [3] in    addition to structural units [G] and [P].-   (5) With conventional vinyl polymers alone, it is difficult to form    high-strength coatings. When employed in combination with a    polyurethane resin, coating strength is ensured (see, for example,    Documents 2 and 3). By contrast, good running durability was    achieved by means of a vinyl copolymer (the binder of the present    invention) in the magnetic tapes of the Examples without combining a    polyurethane resin.

The above results indicate that the binder of the present invention is avinyl copolymer capable of manufacturing magnetic recording media withboth good running durability and good electromagnetic characteristics.

The magnetic recording medium of the present invention can afford bothgood electromagnetic characteristics and running durability, and is thussuitable as a backup tape or the like, of which high reliability isdemanded.

Although the present invention has been described in considerable detailwith regard to certain versions thereof, other versions are possible,and alterations, permutations and equivalents of the version shown willbecome apparent to those skilled in the art upon a reading of thespecification and study of the drawings. Also, the various features ofthe versions herein can be combined in various ways to provideadditional versions of the present invention. Furthermore, certainterminology has been used for the purposes of descriptive clarity, andnot to limit the present invention. Therefore, any appended claimsshould not be limited to the description of the preferred versionscontained herein and should include all such alterations, permutations,and equivalents as fall within the true spirit and scope of the presentinvention.

Having now fully described this invention, it will be understood tothose of ordinary skill in the art that the methods of the presentinvention can be carried out with a wide and equivalent range ofconditions, formulations, and other parameters without departing fromthe scope of the invention or any embodiments thereof.

All patents and publications cited herein are hereby fully incorporatedby reference in their entirety. The citation of any publication is forits disclosure prior to the filing date and should not be construed asan admission that such publication is prior art or that the presentinvention is not entitled to antedate such publication by virtue ofprior invention.

What is claimed is:
 1. A binder composition for a magnetic recordingmedium, which comprises a vinyl copolymer comprising a structural unitdenoted by general formula [G] and a structural unit denoted by generalformula [P]:

wherein, in general formula [G], R^(a1) denotes a hydrogen atom, ahalogen atom, or a methyl group, L^(a) denotes a single bond or adivalent linking group, and A denotes a repeating unit denoted bygeneral formula [a]:

wherein, in general formula [a], each of R^(a11), R^(a12), and R^(a13)independently denotes a hydrogen atom, a halogen atom, or a methylgroup, each of A¹ and A² independently denotes a vinyl monomer residue,n1 denotes an integer of equal to or greater than 1, and * denotes aposition of a bond with the group denoted by L^(a);

wherein, in general formula [P], each of R^(b1) and R^(b2) independentlydenotes a hydrogen atom, a halogen atom, or a methyl group, each ofL^(b1) and L^(b2) independently denotes a single bond or a divalentlinear linking group, each of X^(b1) and X^(b2) denotes a hydrogen atom,a halogen atom, a methyl group, or a polar group selected from the groupconsisting of a sulfonic acid group, a sulfonate group, a carboxylicacid group, a carboxylate group, a phosphoric acid group, a phosphategroup, and a salt thereof, wherein at least either X^(b1) or X^(b2) isthe polar group.
 2. The binder composition for a magnetic recordingmedium according to claim 1, wherein the repeating unit denoted bygeneral formula [a] is a repeating unit denoted by general formula [a1]:

wherein, in general formula [a1], each of R^(a11), R^(a12), R^(a13),and * is defined as in general formula [a], each of A¹¹ and A¹²independently denotes a (meth)acrylate monomer residue, and n2 denotesan integer of equal to or greater than 10 but equal to or less than 500.3. The binder composition for a magnetic recording medium according toclaim 1, wherein the vinyl copolymer further comprises a structural unitdenoted by general formula [1], a structural unit denoted by generalformula [2], and a structural unit denoted by general formula [3]:

wherein, in general formula [1], R¹ denotes a hydrogen atom, a halogenatom, or a methyl group, L¹ denotes a single bond or a divalent linkinggroup, and Y an alicyclic group;

wherein, in general formula [2], R² denotes a hydrogen atom, a halogenatom, or a methyl group, L² denotes a single bond or a divalent linkinggroup, and Z denotes a hydrocarbon group with a carbon number rangingfrom 8 to 50;

wherein, in general formula [3], R³ denotes a hydrogen atom, a halogenatom, or a methyl group, and L³ denotes a single bond or a divalentlinking group.
 4. The binder composition for a magnetic recording mediumaccording to claim 3, wherein the structural unit denoted by generalformula [3] is a structural unit denoted by general formula [6]:

wherein, in general formula [6], R³¹ denotes a hydrogen atom or a methylgroup, X³ denotes —O—, —S—, or a divalent linking group denoted by—N(R³³)—, R³³ denotes a hydrogen atom or an optionally substituted alkylgroup with a carbon number ranging from 1 to 8, and R³² denotes anoptionally substituted alkylene group with a carbon number ranging from2 to 8 or a divalent group in which multiple such alkylene groups arelinked through a linking group.
 5. The binder composition for a magneticrecording medium according to claim 3, wherein the structural unitdenoted by general formula [1] is a structural unit denoted by generalformula [4]:

wherein, in general formula [4], R¹¹ denotes a hydrogen atom or a methylgroup, X¹ denotes —O—, —S—, or a divalent linking group denoted by—N(R¹²)—, denotes a hydrogen atom or an optionally substituted alkylgroup with a carbon number ranging from 1 to 8, and Y¹ denotes analicyclic condensed cyclic group.
 6. The binder composition for amagnetic recording medium according to claim 3, wherein the structuralunit denoted by general formula [2] is a structural unit denoted bygeneral formula [5]:

wherein, in general formula [5], R²¹ denotes a hydrogen atom or a methylgroup, X² denotes a divalent linking group denoted by —(O)m¹-, —(S)m²-,or —{N(R²²)}m³-, each of m¹, m², and m³ independently denotes an integerof equal to or greater than 1, R²² denotes an optionally substitutedalkyl group with a carbon number ranging from 1 to 8, and n denotes aninteger ranging from 12 to
 30. 7. The binder composition for a magneticrecording medium according to claim 1, which further comprises apolyisocyanate.
 8. A magnetic recording medium comprising a magneticlayer comprising a ferromagnetic powder and a binder on a nonmagneticsupport, which comprises at least one layer comprising a binder of whichconstituent component is a vinyl copolymer comprising a structural unitdenoted by general formula [G] and a structural unit denoted by generalformula [P]:

wherein, in general formula [G], R^(a1) denotes a hydrogen atom, ahalogen atom, or a methyl group, L^(a) denotes a single bond or adivalent linking group, and A denotes a repeating unit denoted bygeneral formula [a]:

wherein, in general formula [a], each of R^(a11), R^(a12), and R^(a13)independently denotes a hydrogen atom, a halogen atom, or a methylgroup, each of A¹ and A² independently denotes a vinyl monomer residue,n1 denotes an integer of equal to or greater than 1, and * denotes aposition of a bond with the group denoted by L^(a);

wherein, in general formula [P], each of R^(b1) and R^(b2) independentlydenotes a hydrogen atom, a halogen atom, or a methyl group, each ofL^(b1) and L^(b2) independently denotes a single bond or a divalentlinear linking group, each of X^(b1) and X^(b2) denotes a hydrogen atom,a halogen atom, a methyl group, or a polar group selected from the groupconsisting of a sulfonic acid group, a sulfonate group, a carboxylicacid group, a carboxylate group, a phosphoric acid group, a phosphategroup, and a salt thereof, wherein at least either X^(b1) or X^(b2) isthe polar group.
 9. The magnetic recording medium according to claim 8,wherein the repeating unit denoted by general formula [a] is a repeatingunit denoted by general formula [a1]:

wherein, in general formula [a1], each of R^(a11), R^(a12), R^(a13), andis defined as in general formula [a], each of A¹¹ and A¹² independentlydenotes a (meth)acrylate monomer residue, and n2 denotes an integer ofequal to or greater than 10 but equal to or less than
 500. 10. Themagnetic recording medium according to claim 8, wherein the vinylcopolymer further comprises a structural unit denoted by general formula[1], a structural unit denoted by general formula [2], and a structuralunit denoted by general formula [3]:

wherein, in general formula [1], R¹ denotes a hydrogen atom, a halogenatom, or a methyl group, L¹ denotes a single bond or a divalent linkinggroup, and Y an alicyclic group;

wherein, in general formula [2], R² denotes a hydrogen atom, a halogenatom, or a methyl group, L² denotes a single bond or a divalent linkinggroup, and Z denotes a hydrocarbon group with a carbon number rangingfrom 8 to 50;

wherein, in general formula [3], R³ denotes a hydrogen atom, a halogenatom, or a methyl group, and L³ denotes a single bond or a divalentlinking group.
 11. The magnetic recording medium according to claim 10,wherein the structural unit denoted by general formula [3] is astructural unit denoted by general formula [6]:

wherein, in general formula [6], R³¹ denotes a hydrogen atom or a methylgroup, X³ denotes —O—, —S—, or a divalent linking group denoted by—N(R³³)—, R³³ denotes a hydrogen atom or an optionally substituted alkylgroup with a carbon number ranging from 1 to 8, and R³² denotes anoptionally substituted alkylene group with a carbon number ranging from2 to 8 or a divalent group in which multiple such alkylene groups arelinked through a linking group.
 12. The magnetic recording mediumaccording to claim 10, wherein the structural unit denoted by generalformula [1] is a structural unit denoted by general formula [4]:

wherein, in general formula [4], R¹¹ denotes a hydrogen atom or a methylgroup, X¹ denotes —O—, —S—, or a divalent linking group denoted by—N(R¹²)—, R¹² denotes a hydrogen atom or an optionally substituted alkylgroup with a carbon number ranging from 1 to 8, and Y¹ denotes analicyclic condensed cyclic group.
 13. The magnetic recording mediumaccording to claim 10, wherein the structural unit denoted by generalformula [2] is a structural unit denoted by general formula [5]:

wherein, in general formula [5], R²¹ denotes a hydrogen atom or a methylgroup, X² denotes a divalent linking group denoted by —(O)m¹-, —(S)m²-,or —{N(R²²)}m³-, each of m¹, m², and m³ independently denotes an integerof equal to or greater than 1, R²² denotes an optionally substitutedalkyl group with a carbon number ranging from 1 to 8, and n denotes aninteger ranging from 12 to
 30. 14. The magnetic recording mediumaccording to claim 8, wherein the layer comprises a reaction product ofthe vinyl copolymer and a polyisocyanate.
 15. The magnetic recordingmedium according to claim 8, wherein the layer is the magnetic layer.16. The magnetic recording medium according to claim 8, wherein thelayer is a nonmagnetic layer comprising a nonmagnetic powder and abinder and being positioned between the magnetic layer and thenonmagnetic support.
 17. The magnetic recording medium according toclaim 8, wherein the ferromagnetic powder is a hexagonal ferrite powderhaving an average plate diameter ranging from 10 nm to 50 nm.
 18. Themagnetic recording medium according to claim 8, wherein theferromagnetic powder is a ferromagnetic metal powder having an averagemajor axis length ranging from 20 nm to 50 nm.